The present invention relates to an apparatus and a method for detecting an open circuit fault condition in a common-mode differential signal. A fault detection signal is generated for use by another device to report the fault condition.
Differential signaling has been in existence for many years. For example, teletypes were some of the first equipment to use differential signaling to communicate. One common differential signaling technique utilizes a current loop to send and receive information between a receiver and transmitter.
According to this differential signaling technique, information is represented by current sent in one direction around the loop, or the other direction around the loop. A pulse of current in the loop in one direction may correspond to a logic value of xe2x80x9c1xe2x80x9d, while a pulse in the opposite direction may correspond to a logic value of xe2x80x9c0.xe2x80x9d
Current loops have several advantages over other signaling techniques. For example, data sent utilizing a current loop can travel further than data sent through a common RS-232 interfaces. Current loop differential signaling techniques also provide protection against electrical interference. Additionally, current loop differential signaling techniques can reliably make connections when other communication techniques cannot.
One type of a current loop differential signaling technique is Low Voltage Differential Signaling (LVDS). LVDS is a differential signaling technique commonly used in data transmission systems. LVDS uses relatively low supply voltages; VDD is generally in the range of 2.5 volts and VSS is generally zero. The valid common-mode range for a LVDS receiver is generally between VSS+50 mV and VDDxe2x88x9250 mV. A low voltage differential signal produced by a line driver typically has peak-to-peak amplitudes in the range from 250 mV to 450 mV. The low voltage swing minimizes power dissipation, while maintaining high transmission speeds. Typical transmission speeds are over 100 Mbps (Mega-bits per second).
The present invention is directed to detecting an open circuit fault condition in a differential signal. More specifically, the present invention is directed to providing an apparatus and a method for detecting when a differential signal is floating outside the valid common-mode range, and generating an open circuit fault detection signal. An open circuit fault condition prevents a valid differential signal from being obtained.
Briefly stated, the invention detects the existence of an open circuit fault condition by employing weak current sources to pull the two portions of the differential signal in opposite directions. The invention is also buffered from the differential signal source. Buffered signals are produced in response to the pulled differential signal. Portions of the buffered signal are compared to reference signals. Open circuit fault condition signals are produced when the amplitude difference between portions of the buffered signal and the reference signals meet predetermined criteria. An open circuit fault detection signal is produced from a comparison of open circuit fault condition signals. The open circuit fault detection signal is available to other devices to communicate the existence of an open circuit.
According to another example of the invention, the fault detection occurs with loading, altering, and disturbing the differential signal source.
According to yet another example of the invention, the reference signals are a predetermined amplitude toward the DC common-mode voltage of the differential signal from the local power supply rails.
According to a further example of the invention, the presence of an open circuit fault condition is signaled by a high control signal.